Composites of steel and rubber wherein reinforcing elements of steel or steel alloys are bonded to rubber find wide application in a variety of products which include, for example, conveyor belts, heavy duty hoses, pulley belts and pneumatic tires. The application of such composites in the fabrication of pneumatic tires has become particularly important. That is, there has been an ever increasing use of steel cord to provide greater strength and reinforcement in vehicular tires.
The traditional method for bonding rubber to steel is to plate the steel surface with brass for direct contact to rubber. However, besides being expensive, brass plating suffers from the disadvantages of being unusually sensitive to processing conditions and the rubber composition. As a consequence, alternative methods have been sought with particular attention being given to the use of various organic resin adhesives for application to the steel surface prior to pressing into contact with the rubber component. It has since been found that the well known RFL type organic adhesives which have been used for many years in the bonding of polymeric fibers to rubber provide a satisfactory alternative to the conventional brass plating method.
The basic RFL adhesive composition was first described in U.S. Pat. No. 2,128,229 which issued in 1938 and consists of an aqueous emulsion of a resorcinol-formaldehyde resin and a rubber latex. Although there have been various improvements and additions to this basic formulation over the years, such as are described in U.S. Pat. Nos. 3,817,778 and 3,835,082, these additions and improvements have not fundamentally altered the character of the basic composition and all such variants are generally referred to as RFL type organic adhesives.
Although the use of RFL adhesives as the bonding agent in the fabrication of steel and rubber composites has produced generally satisfactory results, a drawback has recently been observed which is manifested when articles comprised of such composities are subjected to moisture-containing environments during use. That is, it has been found that the bond between the adhesive and the steel surface becomes weakened in the presence of water. When exposure to moisture is prolonged, the steel reinforcing element tends to separate from the adhesive binding it to the rubber and the separated surface becomes exposed to the corrosive effects of the aqueous environment. Eventually, the deterioration caused by this separation and corrosion results in failure of the composite product, for example, a pneumatic tire reinforced with steel cord.
Accordingly, it is a principal object of this invention to substantially improve the hydrolytic stability of the adhesive bond between an RFL type organic adhesive and the surface of steel reinforcing elements employed in composites of rubber and steel.
It is a further object of this invention to improve the adhesive bond between steel and rubber in both wet and dry environments when an RFL type organic adhesive is used as the bonding agent.